Ancient Corals May Provide Record of Rapid Sea Level Rise

With Greenland's glaciers melting and on the move while vast ice sheets in Antarctica continue to shatter, the proportion of water in the seas continues to grow. And with the climate at the poles expected to continue to warm rapidly in coming decades, many researchers are trying to determine how much and how quickly sea levels might rise. Now newly excavated reefs in Mexico may have provided an answer: high and fast.

Geoscientist Paul Blanchon of the National Autonomous University of Mexicoand his colleagues examined the record provided by ancient reefs uncovered during the excavation for Xcaret, a new theme park on the Yucatan Peninsula. By measuring the decay of thorium in the reefs, the researchers estimated their age at roughly 121,000 years old—from a period in the Pleistocene epoch known as the Eemian interglacial, which saw average temperatures that were roughly 3.6 degrees Fahrenheit (2 degrees Celsius) warmer, higher sea levels, and less ice than today.

The buried reefs revealed that sea level rises of as much as two inches (five centimeters) per year resulted in at least a 6.6 foot (two meter) jump in as little as 50 years, based on a series of reefs retreating closer to a receding shore over time. An older reef's tip crested at roughly 10 feet (three meters) above present sea level but a second reef crest farther inland grew 10 feet higher than that, indicating that sea level had risen by as much as 10 feet by the time the latter formed because corals grow nearly to sea level, according to the findings published today in Nature.

"Twenty centimeters (eight inches) of reef accreted or grew in a little over 50 years," Blanchon says. "We found that the first corals that grew in the new reef were up to 1.5 meters [five feet] tall, indicating that sea level had to be at least two meters [6.6 feet] higher than the older reef which grew up to three meters [10 feet] above present sea level."

Other evidence has shown that 14,000 years ago, at the beginning of the current epoch (the Holocene), ice sheet melting led to sea level rises of as much as 49 feet (15 meters) in 300 years. But this find indicates that sea level can rise even faster, most likely from collapsing ice sheets, Blanchon says.

The dating of the reefs by the decay of thorium as well as comparison with similarly aged reefs from the Bahamas remains in question, however, because Blanchon and his colleagues failed to confidently date the first reef.

"Their accuracy is suspect," the researchers admitted in the paper. Yet, other studies have shown that sea levels rose by as much as 5.2 feet (1.6 meters) per century during the Eemian.

This finding "is the first indication that ice sheet collapse caused a sea level jump during the last interglacial," Blanchon says. "If we can find back-stepping reefs during the last interglacial in [Western Australia and other areas], I think we will have a rock-solid case for ice sheet collapse and catastrophic sea level rise." Given the ongoing meltdown in Greenland and Antarctica, that may be a grim presentiment of our own predicament.